1. Field of the Invention
The present invention relates to a passive optical network system, an optical line terminator, and a communication method of a passive optical network system, and in particular to a passive optical network system provided with two or more optical line terminators communicating at different transmission rates in passive optical network systems in which two or more subscriber connection devices have an optical transmission line in common, the optical line terminator in such a passive optical network system, and a communication method of such a passive optical network system.
2. Description of the Related Art
In order for transmitting and receiving large volumes of image signals or data via a communication network, shifting to high-speed and broadband communication network has been promoted also in the access network for connecting the subscribers to the communication network, and introduction of the passive optical network system (hereinafter referred to as a PON) defined by the Recommendations G.984.1 through G.984.3 of International Telecommunication Union Telecommunication Standardization Sector (hereinafter referred to as ITU-T) has been attempted. The PON is a system composed of an optical line terminator (hereinafter referred to as an OLT) to be connected to a host communication network and two or more optical network units (each hereinafter referred to as an ONU) accommodating subscriber terminals (e.g., PCs or phones) connected via a passive optical network including a backbone optical fiber, an optical splitter, and a plurality of feeder optical fibers. Specifically, the PON performs communication in a form of transmitting signal from the terminal (e.g., a PC) connected to each of the ONUs to the OLT as an optical signal with the backbone optical fiber from the feeder optical fiber via the optical splitter while optically (time-division) multiplexing the signal, and the OLT executing a communication process on the signal from each of the ONUs and then transmitting the result to the host communication network or to another ONU connected to the OLT.
Development and introduction of the PON started from a system handling a low-speed signal of 64 Kbit/sec, and introduction of a broadband PON (BPON) transmitting and receiving a fixed-length ATM cell at a maximum rate of about 600 Mbit/sec, Ethernet PON (EPON) transmitting and receiving a variable-length packet of Ethernet at a maximum rate of about 1 Gbit/sec, or a gigabit PON (GPON) standardized by the ITU-T Recommendations G.984.1, G.984.2, and G.984.3 handling a higher-speed signal of about 2.4 Gbit/sec is promoted. Further, now realization of a high-speed PON capable of handling a signal at a rate in a range of 10 Gbit/sec through 40 Gbit/sec is expected. As a measure for realizing such high-speed PONs, there are studied multiplexing methods such as time division multiplexing (TDM) of a number of signals, wavelength division multiplexing (WDM) thereof, or code division multiplexing (CDM) thereof. It should be noted that current PONs adopt the TDM, and the GPON, for example, has a configuration of using different wavelengths between an upstream (from the ONU to the OLT) signal and a downstream (from the OLT to the ONU) signal, and assigning a communication time period of the signal to each of the ONUs in the communication between the OLT and each of the ONUs. Further, the past configuration of processing fixed-length signals is shifting to the configuration of additionally processing burst variable-length signals (burst signals) with which further various types of signals (e.g., sounds, images, and data) can easily be handled. Regarding the high-speed PON in the future, there are studied various multiplexing methods as described above, and the study of applying the TDM is becoming mainstream.
In the form of each of the PONs described above, since the ONUs are implemented in subscriber's houses scattered about various locations, the distances between the OLT and the ONUs are different from each other. Specifically, since the length of the optical fiber (the transmission distance) obtained by adding the backbone optical fiber from the OLT to each of the ONUs and the feeder optical fiber with each other varies, transmission delay between each of the ONUs and the OLT varies, thus there is a possibility that the signals transmitted from the respective ONUs collide and interfere with each other on the backbone optical fiber. Therefore, in each of the PONs, it is arranged that the delay in the output signal of each of the ONUs is adjusted so that the signal outputs from the respective ONUs do not collide with each other after performing measurement of the distance between the OLT and the ONU using a technology called ranging as defined in, for example, chapter 10 of the G.984.3.
Further, it is also arranged that when deciding the signal band allowed for each of the ONUs to transmit based on the transmission request from the ONU using a technology called dynamic bandwidth assignment (hereinafter referred to as DBA), the OLT designates the transmission timing to each of the ONUs so that the signals from the respective ONUs do not collide or interfere with each other in consideration of the amount of delay measured by the ranging described above. In other words, the PON is configured so that the operation of the communication is performed in the condition in which the timing of the signals transmitted and received between the OLT and each of the ONUs is controlled in the system.
According, for example, to the provision of chapter 8.3.3 of G.984.2, in the transmission and reception of the signals between the OLT and each of the ONUs, a guard time composed maximum of 12 bytes for preventing interference, a preamble used for determining a signal discrimination threshold of a receiver in the OLT and clock extraction, burst overhead bytes called a delimiter for discriminating a break of the received signal, and a control signal for the PON (also referred to as an overhead or a header in some cases) added to the data (also referred to as a payload in some cases) at the head of the signal from each of the ONU so that the OLT can distinguish and process the signal from each of the ONUs multiplexed in the backbone optical fiber. It should be noted that since each data (payload) is variable-length burst data, a header called a G-PON encapsulation method (GEM) header for processing the variable-length data is also added to each data at the head thereof.
Further, in the signal from the OLT to each of the ONUs, a frame synchronization pattern for discriminating the head, a PLOAM field for transmitting monitoring information, maintenance information, and control information, and an overhead (also referred to as a header in some cases) called a grant field for designating the signal transmission timing of each of the ONTs are added to the data time-division multiplexed to each of the ONU at the head of the signal transmitted from the OLT to each of the ONU so that each of the ONUs can discriminate and process the signal from the OLT. It should be noted that the GEM header for processing variable-length data is added to the multiplexed data to each of the ONUs similarly to the signal from the ONU. The OLT designates the upstream transmission grant timing (start and stop of transmission) of each of the ONUs to each of the ONUs by byte using the grant field. The transmission grant timing is called a grant. Further, when each of the ONUs transmits data to the OLT with the grant timing, the data is optically (time-division) multiplexed on the optical fiber, and then received in the OLT.